1. Field of the Invention
The present invention is directed to a method for reducing or preventing cracking of direct chill cast ingots, more particularly to a method of direct chill casting using a coolant containing an oxidation inhibitor.
2. Prior Art
Ingots of light metal alloys such as aluminum alloys and magnesium alloys may be produced by direct chill (DC) casting. In conventional DC casting, molten metal is poured into an open-ended mold. The lower end of the mold is initially closed by a platform referred to as a bottom block, and the molten metal pools within the mold. The bottom block is progressively lowered in step with the pouring of the molten metal. The walls of the DC mold are continuously cooled so that a solid skin of metal forms in contact with the mold wall at the level of the surface of the pool of molten metal in the mold. Water is typically used as the coolant for applying to the surface of solid skin that forms in contact with the mold wall. An example of a method of DC casting is described in U.S. Pat. No. 4,071,072, incorporated herein by reference.
It is well established that the tendency of DC cast ingots to cracking is dependent in part on the ingot cooling rate. A high cooling rate increases the ingot shell thickness and strength as the ingot solidifies and thereby lowers the ingot cracking tendency. Attempts to solve the problems of ingot cracking have focused on the ingot casting practice itself to reduce the stresses within the ingots. However, consistent low cracking results have been unachievable to date due to the difficulty in controlling all critical cracking parameters simultaneously. Ingot cracking remains a major recovery problem in ingot production. This problem is particularly severe in high strength alloys such as aluminum alloys used in the aerospace industry.
Accordingly, a need remains for a method of casting metal in a direct chill mold in which the ingot cracking tendency is reduced.